Not Applicable.
The present invention relates to processing of materials, and more particularly to plasma processing.
Plasma processing is widely used to modify surface properties of materials. Thus, plasma is used in fabrication of integrated circuits to perform deposition, etch, cleaning, and rapid thermal anneal. Plasma-based surface processes are also used for hardening of surgical instruments and machine tools, and are used in aerospace, automotive, steel, biomedical, and toxic waste management industries. See, for example, M. A. Lieberman and A. J. Lichtenberg, xe2x80x9cPrinciples of Plasma Discharges and Materials Processingxe2x80x9d (1994), page 1.
A common goal in a plasma-based process design is uniform treatment of the target surface (i.e. the surface treated with plasma). It is desirable to develop systems in which the uniform processing is facilitated.
In some systems, the target article and the plasma move relative to each other, and it is desirable to facilitate precise control of this relative movement. Further, it is desirable to reduce stresses on the target articles thus reducing the possibility of damaging the target articles.
It is also desirable to increase the productivity of plasma processing systems. A plasma processing system is idle when it is being loaded with articles to be processed or when already processed articles are being unloaded. It is desirable to reduce the idle time of the plasma system.
Some embodiments of the present invention provide methods and apparatus for moving the target articles relative to the plasma so as to facilitate uniform processing of the target surfaces. In particular, some embodiments facilitate precise control of the movement of the articles relative to the plasma by reducing accelerations of the articles. Reducing the accelerations also results in reduction of stresses to which the articles are subjected.
The inventor has observed that the velocity of the target article as it moves through the plasma may have to be varied to achieve uniform plasma processing. Consider, for example, the dynamic plasma treatment (DPT) system described in Yu. M. Agrikov et al., xe2x80x9cDynamic Plasma Treatment of HIC (Hybrid Integrated Circuit) Substratesxe2x80x9d, Elektronnaya Tehnika, Ser. 10, 5(71), 1988, pages 30-32, incorporated herein by reference. In that system, a target substrate is moved in and out of the plasma in a chamber maintained at atmospheric pressure. The substrate is moved by a horizontal arm rotating in a horizontal plane. The plasma flows vertically, intersecting the substrate path. The horizontal cross-section of the plasma is smaller than the substrate surface being treated. Therefore, the plasma source moves along the rotation radius to process the whole surface.
Since the substrate points that are located farther from the rotation axis move faster than the points closer to the rotation axis, the points farther from the rotation axis could be exposed to the plasma for less time than the points closer to the axis, resulting in non-uniform processing. One solution to this problem is to vary the angular velocity of the substrate as the plasma source moves along the rotation radius. Thus, when the plasma source is farther from the rotation axis, the angular velocity can be decreased to increase the time that the substrate moves through the plasma.
Another solution is to vary the velocity of the plasma source.
Both solutions need improvement. Thus, varying the angular velocity of the substrate leads to accelerations that make precise control of the angular velocity more difficult to achieve. Further, these accelerations create stresses that may damage the substrate if the substrate is fragile, for example, if the substrate is a semiconductor wafer. Therefore, for this solution, it is desirable to reduce variations of the substrate angular velocity.
Varying the velocity of the plasma source is disadvantageous because accelerations experienced by the plasma relative to immobile ambient gas can change the plasma characteristics and hence make the processing less uniform. Of note, if the processing occurs at atmospheric pressure (as does DPT), even constant-velocity movement of the plasma source can make the plasma difficult to control unless the plasma motion is very slow. Thus, it is desirable to reduce the velocity and acceleration of the plasma source, preferable down to zero.
Accordingly, in some embodiments of the present invention, target surface points that move at different velocities are caused to travel different distances through the plasma so that the faster moving points travel a longer distance. As a result, the time spent in the plasma by faster moving points approaches the time spent by slower moving points. Consequently, the accelerations needed to make the plasma processing uniform are reduced.
In some embodiments, the plasma source is stationary.
In some embodiments, these advantages are achieved as follows. The plasma flow cross-section through which the target article moves is made to have different dimensions in different directions. The target article passes through the plasma multiple times in different directions so that the points moving faster intersect the plasma along a longer dimension of the cross-section than the slower moving points. As a result, uniform treatment can be obtained with less variation of the article velocity.
In some embodiments, the plasma source is stationary. Changing the direction in which the target article intersects the plasma is achieved by rotating the drive that rotates the article so that the article rotates around a first axis which itself rotates around a second axis. The directions change because the article intersects the plasma at different positions of the first axis.
In some embodiments, the idle time of a plasma processing system is reduced by providing two loading devices. While the system processes one article or one lot of articles, one of the loading devices stands ready to unload the system. At the same time, the other loading device is being loaded with the next article or lot of articles to be processed by the system. As soon as the current processing cycle terminates, the processed articles are unloaded into the first device, and the plasma system is loaded from the second device even before the first device is itself unloaded. Meanwhile, the first device carries the processed articles to an appropriate destination, for example, a cassette for semiconductor wafers, unloads the articles, and gets loaded with unprocessed articles. At the same time, the plasma processing system starts processing the articles loaded from the second device, without waiting for the first device. Then the first and second devices switch roles, with the second device waiting to unload articles from the plasma processing system and the first device ready to load unprocessed articles into the system. Because the plasma processing system does not wait for reloading of the first device, the system idle time is reduced.
Other embodiments and variations are discussed below. The invention is defined by the appended claims.